It is known that the autopilot system and the flight control systems of modern transport airplanes generally cooperate with one or more radio altimeters being able to measure the height with respect to the ground. Such a measurement is mainly used at the vicinity of the ground by the autopilot system for the guidance upon an automatic landing, and by flight control systems for carrying out piloting law transitions, for modifying slaving logics of some steerings (horizontal plane for example) or for taking into account the ground effects and piloting objectives associated with the flight phases to be considered.
Because of the commonly used technology (measurement of radio echoes), allowing the height to be deduced with respect to the ground by measuring the propagation time of radio waves, it could occur, more specifically, when the antenna facility becomes impaired or when water flows down on the antennas, that the radio altimeter delivers an erroneous height measurement, being abnormally low.
When the radio altimeter supplies an abnormally low height, the systems receiving such a height use an erroneous piece of information able to result, for the autopilot system, in an erroneous guidance (for instance an early triggering of the guidance of the flare-out), and for the flight control systems in an appropriate development of parameters.
Although the consequences of such a deficiency could be managed by the crew, thanks, more specifically, to usual alarms allowing the effects of the deficiency to be detected, the aim of the present invention is to simplify the operational impact on the systems (for example the use of another radio altimeter that would not have a detected anomaly), providing an advantageous solution for detecting this type of deficiency, including in the vicinity of the ground.
The usual means enabling to detect abnormally low values of a radio altimeter are based on the comparison between two values issued from two completely independent radio altimeters. If a significant difference between the two measures occurs, an alarm is generally emitted, and some systems use the most likely value as a function of the aerodynamic configuration of the airplane (position of the slats and flaps, position of the landing gear, speed of the airplane, etc.).
These logics, although they allow the effects of the deficiency to be managed, only operate when the airplane is provided with two radio altimeters, and can induce a significant operational hindrance (throttling up in approach, diverting to an airport with better weather conditions, etc.).
The present invention aims at solving the above mentioned drawbacks. It relates to a method for automatically detecting an erroneous height value, that is a value with an abnormally low height, being supplied by a radio altimeter mounted on an aircraft, in particular a transport airplane.